High threshold voltage NMOS transistors for low power IC technology

ABSTRACT

Transistors exhibiting different electrical characteristics such as different switching threshold voltage or different leakage characteristics are formed on the same chip or wafer by selectively removing a film or layer which can serve as an out-diffusion sink for an impurity region such as a halo implant and out-diffusing an impurity such as boron into the out-diffusion sink, leaving the impurity region substantially intact where the out-diffusion sink has been removed. In forming CMOS integrated circuits, such a process allows substantially optimal design for both low-leakage and low threshold transistors and allows a mask and additional associated processes to be eliminated, particularly where a tensile film is employed to increase electron mobility since the tensile film can be removed from selected NMOS transistors concurrently with removal of the tensile film from PMOS transistors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application61/161,790, filed Mar. 20, 2009, which is fully incorporated herein byreference.

FIELD OF THE INVENTION

The present invention generally relates to low power semiconductorintegrated circuit technology and, more particularly, to formation oftransistors having low leakage current together with high integrationdensity low power transistors and logic circuitry at very small minimumfeature size.

BACKGROUND OF THE INVENTION

Desired increases in performance and functionality of integratedcircuits have driven designs to very extremely high integration densityand extremely small transistor dimensions. While such designs can offerimprovements in feasible clock and switching speed and reduced signalpropagation time, power requirements, power dissipation constraints andbreakdown resistance for increased numbers of transistors in greaterproximity and operating at increased clock speeds has required reductionof switching voltage threshold of transistors used in low power designs.

However, reduction of switching threshold of logic transistors scaled tosmaller sizes also decreases the off/on resistance ratio of suchtransistors since reduction of dimensions of the conduction channeltends to increase “on” resistance while lower control voltages tend toincrease leakage and decrease “off” resistance. These deviations fromideal switching characteristics tend to increase the power consumptiondue to leakage currents and compensation thereof as well as compromisingnoise immunity and susceptibility to switching errors. Therefore, veryrecent low power integrated circuit designs have included transistorshaving reduced leakage and operating at increased switching thresholdvoltages at strategically located points and/or in particular selectedpaths in the low power circuits to limit the leakage currents which arecharacteristic of other aggressively scaled low power transistorsforming the preponderance of the circuit elements of the integratedcircuit. Only one serially connected transistor of NMOS type is suitablefor a majority of such circuit paths.

However, such an approach has been accompanied by increased processcomplexity and higher cost of manufacture. Specifically, to increase theswitching threshold and decrease leakage of selected transistors whichare of larger size but otherwise process compatible with smaller, lowpower transistors, it has been necessary to increase the concentrationof so-called halo implants surrounding the conduction channel in thetransistors. Doing so has required use of an additional mask whichintroduces additional process complexity and cost while engenderingsources of process error (e.g. overlay and/or lithographic errors) andcompromising manufacturing yield.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a lowleakage transistor which is process compatible with aggressively scaledlow switching threshold transistors that can be formed without use of aseparate mask.

It is another object of the invention to provide an integrated circuithaving substantially identical transistor structures but exhibitingsignificantly different electrical characteristics.

In order to accomplish these and other objects of the invention, anintegrated circuit is provided including a plurality of transistors ofbenchmark design of a first conductivity type and including anout-diffusion sink for impurities in a semiconductor material of thetransistors of benchmark design, and at least one transistor of thefirst conductivity type exhibiting reduced leakage compared withtransistors of benchmark design, wherein a total impurity dose in atransistor of benchmark design and in the adjacent out-diffusion sinkand an impurity dose in an impurity implant of the at least onetransistor are substantially equal.

In accordance with another aspect of the invention, a method of makingan semiconductor device having transistors of a first conductivity typebut exhibiting different electrical characteristics is providedcomprising steps of forming transistors of a first conductivity typehaving an impurity region containing an impurity which increases avoltage threshold of the transistors, forming a blanket film coveringthe transistors, removing a portion of the blanket film from at leastone selected transistor, and out-diffusing an impurity from the impurityregion of transistors of the first conductivity type to remainingportions of the blanket film whereby a voltage threshold of transistorsof said first conductivity type other than said at least one selectedtransistor is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1A and 1B are plan views of exemplary mask portions useful forunderstanding the invention and potential applications thereof,

FIG. 1C is a plan view of a more generalized mask portion illustrating apossible combination of FIGS. 1A and 1B,

FIGS. 2A and 2B are cross-sectional views of two transistorsillustrating a basic principle of operation of the invention,

FIG. 2C is a cross-sectional view of two transistors corresponding tothe generalized mask section of FIG. 1C,

FIGS. 3A and 3B are graphical depictions of the change in on-current,I_(on), and off-current, I_(off), obtained through employment of theembodiment of the invention as illustrated in FIGS. 1A and 2A,

FIGS. 4A and 4B are graphical depictions of the change in thresholdvoltage and I_(off), respectively obtained through employment of theembodiment of the invention as illustrated in FIGS. 1B and 2B, and

FIG. 5 is a graphical plot of I_(on) versus I_(off) comparing electricalcharacteristics of transistors obtained using the invention withelectrical characteristics of transistors obtained with and without haloimplants using a mask which may be omitted through employment of theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1A and 1B,there is shown, in plan view, exemplary portions of masks that may beused to implement the invention and which will be helpful inunderstanding the basic principles of the invention. It should beunderstood that FIGS. 1A and 1B differ only in the types of transistorsformed in the respective quadrants of these Figures and that bothFigures are intended to illustrate the same principle of application ofthe invention and that the invention can be applied to any integratedcircuit design regardless of the structure and required electricalcharacteristics of transistors therein as indicated by the generalizedillustration of FIG. 1C. FIGS. 2A and 2B provide cross-sectional viewsof two transistors corresponding to transistors at transistor sitesshown in FIGS. 1A and 1B, respectively, and illustrating the sameprinciple of operation of the invention. Thus FIGS. 2A and 2B differonly in the labeling of transistor types at transistor sites therein.Similarly, FIG. 2C includes labels corresponding to the more generalizedembodiment corresponding to the mask portion of FIG. 1C. It should benoted that FIG. 1C also illustrates removal of a single portion of thetensile/out-diffusion sink film from all PMOS type transistors togetherwith removal of the tensile/out-diffusion sink film from both longergate/channel NMOS transistors of benchmark threshold and benchmarkgate/channel length, high threshold NMOS transistors. It should beunderstood, however, that less than all types of transistors shown inFIG. 1C may be included in any particular integrated circuit design andthat low leakage transistors preferably include a combination ofincreased channel length and increased switching threshold as is alsoillustrated in FIGS. 1C and 2C.

It will be helpful in coming to an appreciation of the invention toreview some current practices in transistor design for integratedcircuits, particularly where transistors in integrated circuits areaggressively scaled to very small minimum feature sizes approaching thelimits of resolution of lithographic processes. As alluded to above, lowpower integrated circuits are predominantly formed of transistors havinga voltage threshold at about one-half the power supply voltage which maybe on the order of one volt or less and thus generally exhibit a reduced“off” resistance allowing leakage. Accordingly, to reduce or limitleakage, low power integrated circuits require a transistor having ahigher threshold in some paths. However, increase of the thresholdvoltage of a transistor requires an additional mask to, for example,provide an increased impurity concentration in a halo implant which, inturn, can cause gate induced drain leakage, if excessive.

One known alternative for providing low leakage transistors that doesnot necessarily require an extra mask is to increase the gate length(e.g. by 25% to 50% such as from 40 nm for the predominant transistordesign, often referred to as a “benchmark” design, to 50 to 60 nm forlow leakage transistors). However, to be certain that the transistorhaving increased gate length can be strongly turned off, it is desirablethat the transistor have a much higher threshold voltage, V_(th), (oftenreferred to as V_(th) rolloff) than the transistor having the nominal,predominant or benchmark gate length (e.g. 40 nm) which is fixed by theperformance requirements of the design. However, normal, knownsource/drain engineering techniques for producing large V_(th) rolloffdegrade other device characteristics such as drain induced barrierlowering (DIBL) and subVt swing and can even cause susceptibility tosource-drain punch through. In general, approaches to low leakagetransistor designs, as compared with benchmark transistor designs, mayinvolve either greater channel length at a low threshold voltage, a highthreshold voltage at benchmark transistor channel length or, preferably,some combination of increased channel length and increased thresholdvoltage.

In regard to transistor structure design, where transistors areaggressively scaled, it is a known practice to enhance carrier mobilitythrough application of stressed films. A tensile film (commonly anitride although films of other materials can be used) applied, forexample, over a gate structure will apply a compressive force in thegate and, in turn, a tensile force in the transistor channel and improvemobility of electrons which are the majority carrier in NMOS devices.Similarly, a compressive film will apply a tensile force in the gatestructure and a compressive force in the transistor channel to improvethe mobility of holes which are the majority carrier in PMOS devices.Conversely, a compressive force on the channel will inhibit electronmobility and a tensile force on the channel will inhibit hole mobility.Therefore, a blanket application of a tensile film will require removalof that film at PMOS sites using a mask having openings at PMOS siteswhile masking NMOS sites; after which a compressive film may be appliedat PMOS transistor sites, if desired.

Additionally, for a given integrated circuit (IC) design for a givenapplication, most transistors will be of a single design and/or geometry(sometimes referred to hereinafter as a benchmark size, design or gatelength) which can all be formed by a single set of process steps andwhich are optimized for the intended application of the IC. If twodifferent transistor designs are desired in the IC which cannot beformed by the same process steps, block-out masking and separate processsequences must generally be utilized. If, however, transistors having adifferent geometry (e.g. 40 nm polysilicon gate length for the optimizedpredominant type of transistor and a 50 to 60 (50/60) nm polysilicongate length to, for example, increase off resistance and reduce leakagecurrent for other transistors in this example) but which can be formedby the same process sequence are desired (e.g. as shown in the legendsof FIGS. 1A and 2A) or transistors of identical geometry but wheredifferent electrical characteristics are desired (e.g. as shown by thelegends of FIGS. 1B and 2B in which RVt indicates “regular” thresholdvoltage for the predominant number of transistors, LVt indicates areduced threshold voltage compared to RVt and HVt indicates an increasedthreshold voltage compared to RVt as would correspond to a lower leakagecurrent), a single process sequence can be used but only one of thetransistor geometries, at most, can be optimized and some additionalprocess that requires a mask for selectivity is required to obtainacceptable electrical characteristics for the non-optimizedtransistor(s).

In FIGS. 1A and 1B, reference numerals 110 a and 110 b should beunderstood as an assembly of a substrate or body of semiconductormaterial having transistors (corresponding to respective labels) alreadyformed thereon and covered by a blanket tensile nitride (or othermaterial) layer or film 140 (TNF) and a mask for selectively removingportions of the tensile layer. The transistors are substantiallycomplete but for gate source and drain connections and include impurityimplants formed, for example, by a well-known self-aligned technique andso-called halo implants formed adjacent the source and drain regions andunder the sides of the gate structure formed, for example, by well-knownangled implantation techniques. If only a single impurity concentrationis required for respective impurity implants, neither implantationprocess necessarily requires a mask other than block-out masking fordifferent impurity types between the NMOS and PMOS transistors. On theother hand, if different impurity concentrations are required forproducing transistors having different electrical characteristics, anadditional implantation process using an additional mask has beenrequired.

As alluded to above, removing the tensile (e.g. nitride) layer from PMOStransistor sites using mask apertures corresponding to dashed lines 120a, 120 b, is conventional. The invention is preferably implemented bymodification of the mask to include additional apertures 130 a, 130 b,at selected NMOS transistor sites, as well, where high threshold, lowleakage transistors are to be formed. That is, in FIG. 1A, aperture 130a corresponds to the site of a 50/60 nm gate length NMOS transistor(accordingly labeled RVt,N 50/60 nm)) and, in FIG. 1B at least siteslabeled Rvt,N indicate benchmark design transistors having the samegeometry and structural and impurity implant design.

The inventors have discovered that for NMOS devices, the tensile (e.g.nitride) layer 140 can serve as a sink for out-diffusion of boron fromthe halo extension implants 160 of NMOS transistors (indicated by arrow150 in FIGS. 2A and 2B) without significant adverse effect on thetensile properties thereof or improved electron mobility resultingtherefrom. Further, the out-diffusion increases as the concentration ofhalo implant impurities is increased and the rate of out-diffusion ishighly predictable. However, if the sink for boron out-diffusion isremoved, as can be done in a single process for removal of the tensile(e.g. nitride) film (TNF) from the PMOS transistor sites using a single,modified (by provision of aperture 130 a/130 b) mask, out-diffusion inareas where the tensile film has been removed is greatly reduced, if notsubstantially eliminated; leaving a higher concentration of boron in thehalo implant substantially unchanged. By the same token, removal of thetensile film also serves to reduce electron mobility in the transistorslocated at sites 130 a/130 b, tending to reduce leakage, as well.

Therefore, in accordance with the invention, a higher concentration ofboron impurities which is substantially optimized for high thresholdvoltage, low leakage transistors may be implanted in an angled implantstep common to all NMOS transistors in the IC and reduced to theconcentration that is substantially optimized for the predominant orbenchmark design transistor by well-controlled out-diffusion into thetensile film at benchmark design transistor sites but leaving the higherimpurity concentration substantially intact at sites of low-leakagetransistors. Thus, by designing and providing a heavier halo implantimpurity dose 160 which may be optimized for the high V_(th), lowleakage transistor, providing aperture 130 a, 130 b, for removal of thetensile (e.g. nitride) film at such locations and the PMOS transistorsites and removing the tensile film as shown at 145 a, 145 b, (whichalso reduces electron mobility in those NMOS transistors; furtherreducing leakage) and heat treatment, out-diffusion of boron into thetensile (e.g. nitride) film can be caused at benchmark transistor sitesto reduce the boron concentration in the halo implants 160′ to asubstantially optimum level in accordance with performance requirementswhile leaving boron concentration of halo implants at low leakagetransistor sites substantially unchanged since there is no remainingtensile (e.g. nitride) film to provide a sink for boron out-diffusion atlow leakage transistor sites. In other words, by formation of respectivebenchmark design and low leakage design transistors in accordance withthe invention, the total halo implant impurity dose in the halo implantand out-diffused impurity in the tensile film or respective benchmarkdesign transistors is substantially equal to the halo implant dose in alow-leakage, high threshold transistor. In other words, compared withprior art integrated circuit chips, the overall boron content of a chipwill be significantly greater in a chip manufactured in accordance withthe invention and substantially that which would be required if alltransistors were formed with an increased V_(th) through increased boronconcentration in the halo implant (as is, in fact, the case at anintermediate stage of manufacture before the excess boron in benchmarkdesign transistors is out-diffused and rendered ineffective to causeincreased V_(th)) rather than boron concentration in halo implants beingincreased only at sites of low leakage design transistors.

Conversely stated, as compared to the benchmark transistor design,I_(off) of selected transistors having increased gate length, asdiscussed above in connection with FIGS. 1A and 2A, can be dramaticallyreduced while essentially having no effect on the intended electricalproperties of the benchmark design transistors and a desired, specifiedI_(off) can be closely approached for any desired gate length, asgraphically illustrated in FIG. 3A. Further, even though electronmobility is reduced by removal of the tensile (e.g. nitride) film (whichcould theoretically be replaced) I_(on) of low leakage transistorsremains acceptably high while I_(on) of benchmark design transistors isnot affected, as graphically illustrated in FIG. 3B.

For transistors of the identical geometry as discussed above inconnection with FIGS. 1B and 2B, as shown in FIG. 4A, the thresholdvoltage, V_(th), can be raised substantially by as much as 80 mV due toremoval of the out-diffusion sink. Further, as shown in FIG. 4B, byremoval of the out-diffusion sink and substantially preventing boronout-diffusion from the halo implant, I_(off) can be reduced by up tosubstantially an order of magnitude (e.g. by 90%).

FIG. 5 shows a plot of I_(off) as a function of I_(on) (e.g.corresponding to off/on resistance ratios) for transistors of variousdesigns to compare the results of transistors formed in accordance withthe invention to corresponding benchmark transistor designs andtransistors formed using an additional mask to increase halo implantimpurity concentrations have much reduced leakage. It can be clearlyseen that both I_(off) and I_(on) are relatively large (corresponding tohigh leakage) for benchmark design transistors, as shown by line 51 butare reduced by virtue of the invention, as shown by line 52 indicatinggreatly reduced leakage. Moreover, the cluster of values developed inaccordance with the invention, shown by dashed line 53, is entirelycomparable to and possibly somewhat better than the values obtained byforming the transistors using an additional mask and implantationprocess to increase halo implant dose as in the prior art whileeliminating a complex lithographic process which may compromisemanufacturing yield.

In view of the foregoing, it is clearly seen that the invention providesa simplified and more economical process for formation of low leakagetransistors using substantially the same processes used for benchmarktransistors in integrated circuits of arbitrary design and which areessentially required for acceptable performance and operation oflow-power integrated circuits. Further, the invention provides forachieving much different electrical characteristics of transistorshaving substantially identical geometry and structures on a single chipand using a common process sequence. It should be understood that whilethe use of a tensile film as an out-diffusion sink is the mosteconomical, advantageous and convenient application of the inventionpresently contemplated by the inventors, the stress level in thefilm/out-diffusion sink is of no importance to the successful practiceof the invention and any film or material deposit that can bemodified/removed at selected transistor sites can be used as anout-diffusion sink. It should also be appreciated that the invention, inaccordance with its most basic principles, is not limited to alterationof electrical characteristics of NMOS transistors or boron but can beemployed in regard to any impurity exhibiting a relatively highdiffusion rate and any structure including such an impurity, such as inextension implants adjacent source and drain structures of PMOStransistors which typically include boron and where impurities of lowerdiffusivity (e.g. phosphorus and/or arsenic) are used for halo implants.In this regard, the invention may be useful in limiting diffusion ofboron within a transistor channel by providing a diffusion sink intowhich higher diffusivity impurities may preferentially out-diffuse.

While the invention has been described in terms of two embodimentsillustrating general applicability of the invention to all transistorand integrated circuit designs, those skilled in the art will recognizethat the invention can be practiced with modification within the spiritand scope of the appended claims.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent is as follows:
 1. A method of making asemiconductor device having transistors of a first conductivity type butexhibiting different electrical characteristics, said method comprisingsteps of forming a plurality of transistors of a first conductivity typehaving a channel region containing an impurity which increases a voltagethreshold of said plurality of transistors of said first conductivitytype, each of said plurality of transistors of first conductivity typehaving substantially the same total impurity dose in said channelregion, wherein said total impurity dose in said channel region of saidplurality of transistors of said first conductivity type comprises ahalo implant, forming a blanket film covering said plurality oftransistors of said first conductivity type, removing one or moreportions of said blanket film from at least one selected transistor ofsaid plurality of transistors of said first conductivity type, anddiffusing an impurity from said channel region of transistors of saidfirst conductivity type which are covered by remaining portions of saidblanket film into material adjacent said channel region, whereby avoltage threshold of said plurality of transistors of said firstconductivity type other than said at least one selected transistor isreduced, and wherein said diffusing step removes said halo implant fromsaid channel region of said transistors of said first conductivity typewhich are covered by remaining portions of said blanket film, whereinafter said diffusing step, a total impurity dose in a channel region ofa covered transistor of said plurality of transistors of a firstconductivity type and material adjacent said channel region issubstantially equal to a total impurity dose in a channel region of saidat least one selected transistor from which said blanket film wasremoved but differently distributed.
 2. The method as recited in claim1, including the further steps of forming a plurality of transistors ofa second conductivity type, forming a blanket film covering saidplurality of transistors of said second conductivity type, and removingone or more portions of said blanket film from at least one selectedtransistor of said plurality of transistors of said second conductivitytype.
 3. The method as recited in claim 2, wherein said step of removingone or more portions of said blanket film from said at least oneselected transistor of said plurality of transistors of said secondconductivity type includes said step of removing one or more portions ofsaid blanket film from said at least one selected transistor of saidfirst conductivity type.
 4. The method as recited in claim 2, whereinsaid plurality of transistors of said second conductivity type are NMOStransistors.
 5. The method as recited in claim 1, wherein said pluralityof transistors of said first conductivity type are NMOS transistors. 6.The method as recited in claim 1, wherein said step of diffusing animpurity includes diffusion of boron.
 7. The method as recited in claim1, wherein said step of forming a plurality of transistors of a firstconductivity type includes forming transistors of different gate/channellengths.
 8. The method as recited in claim 2, wherein said step offorming a blanket film covering said plurality of transistors of saidfirst conductivity type includes said step of forming a blanket filmcovering said plurality of transistors of said second conductivity type.9. The method as recited in claim 1, wherein said step of forming aplurality of transistors of a first conductivity type includes formingtransistors of equal gate/channel lengths.